The present invention relates to a transmission line correction device.
Transmission lines are employed generally in the telecommunications field, their purpose being to transfer an electrical signal from one point to another point. This can for example involve transmitting a video signal from a camera head to a mobile outside broadcast vehicle, or, again, between two blocks of buildings.
Every signal travelling along a transmission line becomes deformed. Thus, every transmission line causes the signal it is carrying to be attenuated, and the longer the line is, the greater the attenuation. For lines of the same type and length, for example coaxial cable constituted by the same elements, there is nevertheless a variation in attenuation from one line to an other. This is due to the fact that the elements constituting such lines are not strictly identical and nor are they associated in a strictly identical manner.
This difference in attenuation from one line to another is present both in the case of a DC signal, with zero frequency, as well as for signals of any particular frequency f. On the other hand, when the frequency f of a signal carried on a line increases, there is also an increase in signal attenuation.
Those skilled in the art know that when one considers any signal, the latter comprises a DC component and components at various frequencies. It is thus necessary, for such a signal, to simultaneously correct the DC component attenuation and the attenuation of the components at different frequences.
Various systems are known in the art which are suitable for providing such correction. In what follows, we shall use the expression "amplitude correction" to denote amplitude correction applied to a DC signal or to the DC component of any signal whatsoever. Similarly, we shall use the expression "correction for frequency" to denote the process of correcting the amplitude of a frequency signal for the components at different frequencies of any given signal.
Certain correction systems work on the basis of switching in so called correction cells. Such systems correct defects in amplitude and frequency brought about by propagation. These correction cells are built up from discrete components such as resistors, self-inductances or capacitors. Such correction systems must be located at precise points in the transmission line as they are designed to be able to correct defects in the line over a given length. The individual components constituting such cells are adjusted manually. This is disadvantageous as such adjustments are time consuming and difficult to get right. It is hence practically impossible to rapidly and cheaply adjust a system composed of several transmission lines in parallel.
Other systems use automatic correction using closed-loop feedback control, the reference value for which is derived from the actual signal. Automatic correction is only a correction of amplitude. In order to provide correction for frequency, a filter needs to be introduced into the correcting device. This filter, which is itself composed of discrete components suffers from the disadvantage of requiring manual adjustment for a given transmission line. The adjustment for one line will not be the same as that for another, even if the lines are of the same type.